In order to reduce emissions of air pollutants, especially nitrogen oxides (NOx), from gas turbine power generation plants, emissions regulations and air permitting standards are becoming increasingly stringent, particularly in active market areas (e.g. California). Selective catalytic reduction (SCR) is one technology that is currently being used to reduce emissions of NOx to acceptable permit levels. One reducing agent that is commonly used in SCR systems is ammonia (NH3).
Ammonia, typically in gaseous form, is injected into the exhaust flow path by an injection grid where it reacts with the NOx, facilitated by a catalyst, to convert the NOx into non-harmful substances, namely, nitrogen and water. Common reactions include:4NO+4NH3+O2→N2+6H2O2NO2+4NH3+O2→3N26H2ONO+NO2+2NH3→2N2+3H2O
Ammonia reacts with NOx in a substantially equimolar manner. It is important to supply an appropriate amount of ammonia to the exhaust flow. On one hand, too little ammonia leads to unacceptably high NOx levels exhausted to the atmosphere. On the other hand, too much ammonia increases the likelihood that ammonia may pass through an SCR unit without reacting (known as ammonia slip), potentially reaching environmentally harmful levels in the final exhaust. In either case, violations of regulatory requirements may occur, resulting in penalties.
NOx emissions levels are measured and recorded by a Continuous Emissions Monitoring System (CEMS) disposed in the exhaust stack downstream of the SCR unit to evaluate air permit compliance. However, there is a large inherent time lag for most CEMS to provide feedback to the system controller, and it is difficult to accurately predict the appropriate flow to the SCR during gas turbine ramps (i.e., changes in load), leading to a mismatch between the amount of ammonia supplied relative to the amount of NOx in the exhaust flow. However, by the time the mismatch is detected by the CEMS, unacceptable amounts of harmful pollutants may have already been released to the atmosphere.
Thus, there is a need for a system and method that can minimize such concerns.